1. Field of the Invention
The invention relates to a device for sensing a rotary member such as a water meter turbine.
2. Description of the Prior Art
Most water meters comprise a mechanically driven totalizator, i.e. the rotation of the turbine drives index rollers of the meter via a mechanical and/or magnetic transmission.
It is also known in the art to sense rotation of the turbine using a proximity sensor facing a rotary member fastened to the turbine and adapted to sense the passage of an eccentric mark on the rotary member. The proximity sensor may use an inductive process, for example, in which case the mark consists of a material whose magnetic and/or electrical characteristics differ from those of the remainder of the rotary member. FIG. 1 illustrates this kind of sensing. The rotary assembly consists of a turbine 1 of a flowmeter, not shown, and a disk 2 fastened to the turbine 1. When a fluid such as water flows, the turbine 1 and the disk 2 rotate about an axis XX′. The rotation speed of the rotary member is directly related to the instantaneous flowrate of the fluid. Two proximity sensors L0 and L1 in a plane P perpendicular to the axis XX′ and in two radial directions relative to the axis XX′ are sensitive to the proximity of a mark 5 that is fastened to the disk 2 and eccentric to the rotation axis XX′. It follows that when the rotary member rotates, the response of the proximity sensors L0 and L1 varies as a function of the position of the mark 5. For example, the two proximity sensors L0 and L1 are magnetic coils shunted by capacitors, thereby forming two oscillatory circuits disposed in two opposite radial directions. The disk 2 is made of a non-metallic material such as a molded plastic and the mark 5 is a metallized radial sector of the disk.
A sensing system of the above kind gives rise to certain problems, however, because it has a series of parameters that vary as a function of time. For example, for a water meter integrated into a calorimeter, these parameters include the temperature of the water, which may cause the characteristics of the sensor to vary, the supply voltage of the sensing circuit, especially in the case of a battery power supply, and the varying distance between the proximity sensor and the rotary member resulting from the turbine lifting off at high speeds. There is also a series of parameters that vary from one sensor to another and are difficult and costly to control in the context of mass production; for example, for an inductive sensor, these parameters include in particular values of the coil and of its quality coefficient, which leads to the need to sort the sensors or to calibrate each sensor.
One prior art solution to the problems referred to above is described in U.S. Pat. No. 5,187,989. The rotation sensing device described in that document comprises at least two proximity sensors adapted to sense the passage of a mark fastened to the rotary member. The device analyzes the evolution of the signal delivered by a first sensor. As soon as the mark has been sensed by the sensor, the number of turns index is incremented and the second sensor is selected, deselecting the first sensor that has just sensed the mark; the parameters for analyzing the evolution of the signals from the second sensor are then reset; there is obtained in this way an adaptive system that avoids problems associated with variation of the parameters.
The use of a solution of the above kind gives rise to certain problems, however.
This is because the device of this solution uses a complex electronic circuit, resulting not only in a relatively long processing time but also in high power consumption.